I do not think that what you suggest is a good idea. If you just switch the filament out for two output tubes, the bias is still set for 4 output tubes and the two remaining tubes will be biased too high. Also - at the end of the filament line are the filament wires for the two driver tubes. Switching out output tubes would also put out the lights for the two driver tubes with the filaments wired they way they are now. IMHO filament switches would add unnecessary complexity to the filament wiring.

Peterh,Don't over-think this.Pull the back pair of output tubes -- power off of course -- then re-bias the front tubes. You'll drop to about 65 WPC but that's plenty for most speaks, anyways. No soldering nor un-soldering required. The filament end of the power supply will not be looking at a short if you do this and is not a worry. Trust me, you won't break anything. But dial down the front bias accordingly.

But wait! I think it's not about horsepower so much as torque! Whilst 4 tubes gives you more top end power (akin to horse power, if you will), Isn't there a benefit down low (akin to torque?) in dynamics?

Tube Nube wrote:But wait! I think it's not about horsepower so much as torque! Whilst 4 tubes gives you more top end power (akin to horse power, if you will), Isn't there a benefit down low (akin to torque?) in dynamics?

Dave, have you any experience going from 2 to 4 tubes per amp?

Yes, I have. In our week or two of summer here in the Bitterroots I will pull the back tubes out, just to make everybody a bit cooler. At the SPL levels I listen at, I can hear no difference. Just, when you do a tube swap like that, the plate volts go up, so you must, must, must re-check the bias when you do it. Likewise, when you re-install the back tubes. Otherwise, it's a no-brainer

The goal with switching off filament for 2 tubes is to save tubes and sockets from wear. Itwould also make the amp a "switchable 60 or 125w amp"The need for re-bias is there whatever you do unless extra complexity is added that connects / disconnects a pot that adjusts the bias when switching filament power. And the fact thatcurrent filament wiring prevents break of filament to two tubes is a weak argument that is cured by a tag strip.

This is only a suggestion, i have no M-125 myself, just wanted to contribute something that mightbe of value.

Peterh,If you're only doing this once or twice a year due to external temperatures or to save on your electricity bill nothing will be unduly stressed. Any time you pull or replace an output tube, it's a good time to re-tension the sockets anyway, then give them a short squirt of de-oxit before re-installing. This is especially important if you're changing out different tube types. Not all output tube pins are created equal in terms of diameter or circumference. The Russian pins are slightly fatter than the US-made Tung-Sols; I have measured them. So if you've been using KT-120s, for example, then replace them with British KT-88s or US-made TS 6550s, re-tensioning the socket holes is vital unless you enjoy fireworks as a couple of tubes red-plate and sink your bank account.I am also told by a retired Fluke engineer and have mentioned elsewhere on this forum, gold pins and zinc sockets or vice-versa do not make for a happy conductive circuit. Zinc to zinc or gold to gold, but never the twain should meet.I should add that all the filament voltage coming from the transformer will see in an empty socket is an open circuit. No need for a switch. It already knows it has nowhere to go, so it just sits in the transformer.